Specifying Well Beyond the Numbers

Defense-related applications have long been guided by challenging requirements, both mechanically and electrically. Whether for a ground-based system or for use in the air, at sea, or even in outer space, the “mil specs” for military system have defined some rigorous requirements. They have detailed performance levels that must be maintained for a device, a component, or a system over wide temperature ranges—with shock, under vibration, and while encountering various other conditions. Certainly, more than a few designers tasked with meeting these requirements have thought that at least some of them must have been typographical errors—or even psychological misgivings—on the part of the system specifier.

For the most part, military-level specifications are well conceived. The devices and components that are designed and fabricated to meet these requirements generally do not fail when in those “impossible” operating environments, but provide the levels of performance that make radar, and electronic-warfare (EW), and other defense-related systems provide the results that help this world function. The specifications often seem impossible at first, especially to the amplifier or filter designer responsible with reaching and exceeding a startling list of numbers. In some cases, accepting the specifications is simply act of faith.

Circuit materials and their electrical specifications, for example, are considerably different than the specifications that compare the performance levels of many of the components formed from those components, such as filters. Many readers may even remember the power levels and frequency sweep settings on the last signal generator they may have adjusted, or how the controls were set on a recent vector network analyzer (VNA) enlisted to evaluate a microwave amplifier. But most readers will probably admit that it has been a while (if ever) since the last time that they were performing measurements on different PCB materials for relative dielectric constant, or coefficient of thermal expansion (CTE), or some of the other parameters that are only used to characterize PCB materials.

Although many RF/microwave engineers count on the results of such measurements to use those PCB materials in a component, circuit, or system, they could not recreate many of those PCB material measurements. The dielectric materials used in PCBs—and, at the other end of the design, the materials used to package and protect a design—must be measured and specified. The accuracy of those measurements is absolutely essential to the next design step, which is using those numbers in a commercial computer-aided-engineering (CAE) program to simulate a component, circuit, or system.

Fortunately, those who characterize the materials that serve as building blocks for this industry are quite good at what they do, and this editorial at least serves as a note of recognition for the difficult tasks they perform. Determination of a material specification routinely taken for granted, relative dielectric constant, must be accurate. It must remain with the tolerances specified for it in order for some designs (such as antennas) to even come close when all these values and specifications are entered into a commercial CAE simulator. It is really the talents of these suppliers that are to be recognized. The consistent quality of their measurements turns into published PCB material specifications that can be trusted and literally built upon.